Partialy applying multiresolution modifier
Old question but still I know at least a work around for this.

If e.g. you want to get rid of the levels 1 to 3 and keep only 4 to 7 you can do this:


Duplicate model
Go to level of detail you want as lowest (e.g. 4)
Apply Modifier
Add new modifier and subdivide till you have the same subdivision as the original one (in this case subdivide 3 times)
Add a Shrinkwrap modifier and set both Meshes to the highest level
Apply Shrinkwrap modifier


Done.